Polyunsaturated fatty acids (PUFAs) play many roles in the proper functioning of all life forms. For example, PUFAs are important components of the plasma membrane of a cell, where they are found in the form of phospholipids. PUFAs are necessary for the proper development of the infant brain, as well as for tissue formation and repair in mature mammals.
A number of enzymes, most notably desaturases and elongases, are involved in PUFA biosynthesis (See, FIG. 1). Desaturases catalyze the introduction of unsaturations (e.g., double bonds) between carbon atoms within the fatty acid alkyl chain of the substrate. Elongases catalyze the addition of a 2-carbon unit to a fatty acid substrate. For example, linoleic acid (LA, 18:2n-6) is produced from oleic acid (OA, 18:1n-9) by a Δ12-desaturase. Eicosadienoic acid (EDA, 20:2n-6) is produced from LA by a Δ9-elongase. Dihomo-γ-linolenic acid (DGLA, 20:3n-6) is produced from EDA by a Δ8-desaturase (See, FIG. 1). Arachidonic acid (ARA, 20:4n-6) is produced from DGLA by a Δ5-desaturase (See, FIG. 1).
A number of important long-chain PUFAs are known in the art. For example, one of the most important long-chain PUFAs is eicosapentaenoic acid (EPA). EPA is found in fungi and in marine oils. A second important long-chain PUFA is docosahexaenoic acid (DHA). DHA is most often found in fish oil and can also be purified from mammalian brain tissue. A third important long-chain PUFA is ARA. ARA is found in filamentous fungi and can also be purified from mammalian tissues including the liver and the adrenal glands.
ARA, EPA and/or DHA, can be produced via either the alternate Δ8-desaturase/Δ9-elongase pathway or the conventional Δ6-desaturase pathway (See, FIG. 1). Elongases active on substrate fatty acids in the conventional Δ6 pathway for the production of long-chain PUFAs, particularly ARA, EPA and DHA, have previously been identified. The conventional Δ6-desaturasepathway for converting LA to DGLA and alpha-linolenic acid (ALA) to ω3-eicosatetraenoic acid (ω3-ETA) utilizes the Δ6-desaturase enzyme to convert LA to gamma-linolenic acid (GLA) and ALA to stearidonic acid (SDA); and a C18-elongase enzyme to convert GLA to DGLA and SDA to ω3-ETA. However, in certain instances, the alternate Δ8-desaturase/Δ9-elongase may be preferred over the conventional Δ6-desaturase pathway. For example, if particular residual omega-6 or omega-3 fatty acid intermediates, such as GLA or SDA, are not desired during production of DGLA, ARA, ω3-ETA, EPA, ω3-docosapentaenoic acid (DPA) and/or DHA, the alternate Δ8-desaturase/Δ9-elongase pathway may be used as an alternative to the conventional Δ6-desaturase pathway to bypass GLA and SDA formation. Δ8-desaturases are useful in this pathway because they desaturate a fatty acid between the eighth and ninth carbon atom (numbered from the carboxyl-terminal end of the molecule) and can, for example, catalyze the conversion of ω6-eicosadienoic acid (EDA) to DGLA and/or ω3-eicosatrienoic acid (ω3-ETrA) to ω3-ETA. Therefore, there is a need in the art for new sources of Δ8-desaturases that can be used in the production of long-chain PUFAs.